Regulation of the number of functional voltage-sensitive Ca⁺⁺ channels on PC12 cells by chronic changes in membrane potential

The properties of the various types of voltage-sensitive Ca⁺⁺ channels (VSCC) are becoming increasingly well characterized, but the mechanisms which control the number and types of channels expressed by cells are virtually unknown. To study the regulation of VSCC in neuronal cells we have used PC12 pheochromocytoma cells. Binding of [³H]nitrendipine was used to determine the number of dihydropyridine-sensitive channels, and the uptake of ⁴⁵Ca⁺⁺ was used to determine the functional state of VSCC on the cell surface. Prolonged depolarization by elevation of extracellular K⁺ caused concomitant time and concentration-dependent decreases in both [³H]nitrendipine binding and depolarization-dependent uptake of ⁴⁵Ca⁺a+. Changes in binding and ion flux plateaued at about a 50% decrease with 3 days of depolarization and an extracellular K⁺ concentration of 50 mM. Return of the cells to normal K⁺ caused the recovery of both [³H]nitrendipine binding and ⁴⁵Ca⁺⁺ uptake within 24 hr. Measurements of the intracellular free Ca⁺⁺ concentration determined that it remained elevated for several hours with K⁺ depolarization, but returned to normal within 15 hr. Growth of the cells with a concentration of ionomycin, which caused a similar increase in intracellular free Ca⁺⁺, also caused a loss of [³H]nitrendipine binding sites. Thus, it appears that the number of functional VSCC can be regulated by changes in intracellular Ca⁺⁺ such as those associated with prolonged depolarization. However, because Ca⁺⁺ channel number remained depressed while intracellular free Ca⁺⁺ returned to normal, other mechanisms controlling channel number also must be involved.